Professor Brian Cox takes a global journey in search of the energy source of the future.
Called nuclear fusion, it is the process that fuels the sun and every other star in the universe.
Yet despite over five decades of effort, scientists have been unable to get even a single watt of fusion electricity onto the grid.
Brian returns to Horizon to find out why. Granted extraordinary access to the biggest and most ambitious fusion experiments on the planet, Brian travels to the USA to see a high security fusion bomb testing facility in action and is given a tour of the world's most powerful laser.
In South Korea, he clambers inside the reaction chamber of K-Star, the world's first super-cooled, super-conducting fusion reactor where the fate of future fusion research will be decided.
Nuclear fusion is nature's power source. From the Sun to the most distant stars, the energy that lights up the Universe is released by sticking hydrogen nuclei together to make helium.
Since hydrogen is the most abundant element in the Universe, it seems sensible to ask whether we might endeavor to do the same and power ourselves out of our serious energy crisis by building stars on Earth.
The problem of course is that stars are big and hot; the Sun is the size of a million Earths, and burns six hundred million tonnes of hydrogen fuel every second.